Biohydrogen production from organic waste addresses today's two most pressing problems: soaring energy demand and environmental pollution. Microorganisms are capable of producing hydrogen via either photosynthesis or preferably through fermentation (Matsunaga et al., 2000). Organic pollutants are anaerobically converted to methane in two distinct stages: acidification and methanogenesis. Acidification produces hydrogen as a by-product which in turn is used as an electron donor by many methanogens at the second stage of the process (Fang and Liu, 2002). Separation of the two stages is feasible for hydrogen collection from the first stage. The second stage is further used for treatment of the remaining acidification products, which includes mainly volatile fatty acids.
The continuously stirred tank reactor (CSTR) has been the most widely used system for continuous hydrogen production (Li and Fang, 2007). Since in a CSTR biomass solids residence time (SRT) is the same as the hydraulic retention time (HRT), its concentration in the mixed liquor is highly affected by the recommended HRT of 1-12 h which is optimal for high hydrogen production rates (Li and Fang, 2007). The maximum specific growth rate (μmax) for mixed culture of 0.333 h−1 (Horiuchi et al., 2002) corresponds to an SRTmin of 3.0 h.
However, high dilution rates result in a marked decrease in biomass content in the reactor due to severe cell washout and system failure (Wu et al., 2008). Decoupling of SRT from HRT in hydrogen bioreactors has been achieved primarily by using biofilms on several media including synthetic plastic media and treated anaerobic granular sludge (Das et al., 2008), activated carbon, expanded clay and loofah sponge (Chang et al., 2002), glass beads (Zhang et al., 2006) and membranes (Vallero et al., 2005). Problems with the development of methanogenic biofilms on the carrier media adversely impact process stability, which is critical for sustained hydrogen production. Moreover, membranes have not shown many advantages in terms of volumetric hydrogen yield and are also prone to fouling in such a reductive environment.
Therefore, it would be very advantageous to provide a method and integrated system for hydrogen and methane production from industrial organic wastes and biomass which decouples the solids residence time (SRT) from the hydraulic retention time (HRT) in order to avoid some of the aforementioned limitations.